Polypropylene reaction processes involve adding propylene, a polypropylene forming catalyst, and other optional compounds, to a polymer reactor and producing polypropylene product. The product emerging from the reactor contains various contaminants, including unreacted propylene, that must be removed in order to obtain an acceptable polymer.
Propylene feedstreams for the production of polypropylene, as well as for the production of numerous other products, have traditionally been obtained through cracking of hydrocarbons. Non-catalytic cracking processes are described, for example, in Hallee et al., U.S. Pat. No. 3,407,789; Woebcke, U.S. Pat. No. 3,820,955, DiNicolantonio, U.S. Pat. No. 4,499,055 and Gartside et al., U.S. Pat. No. 4,814,067. Catalytic cracking processes are described, for example, in Cormier, Jr. et al., U.S. Pat. No. 4,828,679; Rabo et al., U.S. Pat. No. 3,647,682; Rosinski et al., U.S. Pat. No. 3,758,403; Gartside et al., U.S. Pat. No. 4,814,067; Li et al., U.S. Pat. No. 4,980,053; and Yongqing et al., U.S. Pat. No. 5,326,465.
A particular process of producing propylene streams is described in U.S. Pat. No. 5,714,662 to Vora et al. In this patent, an olefin product is first made by the catalytic conversion of a methanol feedstream. Then, a crude grade propylene stream is recovered from the olefin product as an overhead stream of a distillation column, with part of the crude propylene stream being used as feed to an etherification unit to form diisopropyl ether. A separate portion of the crude propylene stream is passed to a propylene fractionation zone to produce a high purity propylene stream.
As discussed in U.S. Pat. No. 6,271,319 to Baker et al., high purity propylene is generally used as feed to a polypropylene reaction process. This patent further notes that the presence of propane is typically a problem in polypropylene reaction systems, and the removal of propane is particularly difficult. Thus, a method of removing propane from the reaction system using a membrane is described.
The difficulty in removing propane from propylene streams means that extensive separation equipment is required to obtain a polymer grade propylene feedstream. In turn, such equipment is expensive and substantially increases the cost of the polymer grade propylene feedstream. It would, therefore, be beneficial to obtain lesser grade propylene feedstreams that would be effective in the manufacture of polypropylene. It would also be beneficial to recover unreacted contaminants such as propylene from polymer product.